Heretofore it has been known to adjust the temperature of molten glass flowing through a forehearth by means of Joule effect heating. Molten glass enters a forehearth at a temperature exceeding that at which it is to be worked into the end product of the glass forming operation. It is cooled as it flows along the forehearth to the point of delivery to forming apparatus even at the maximum rate of draw, without the addition of heat, the glass is at a lower temperature and thus a greater viscosity than is optimum for the glass forming operation. Supplemental heat is applied to the forehearth to retard the rate of cooling of the molten glass or even raise its temperature to the desired working temperature at the point from which it is delivered to forming apparatus.
Electric heating by Joule effect has been employed in various arrangements which frequently seek to establish thermal zones longitudinally of the forehearth, either by the passage of the current supplied across those zones or longitudinal of those zones. Current flow transverse of the stream of molten glass in a forehearth to apply Joule effect heat to longitudinally spaced zones in the molten glass is shown in Henry U.S. Pat. No. 1,928,288 of Sept. 26, 1933 for "Forehearth for Molten Glass and Method of Controlling the Temperature of the Glass Therein". Longitudinal flow of current in the molten glass flowing in a forehearth is shown in Nuzum U.S. Pat. No. 3,198,619 of Aug. 3, 1965 for "Tubular Forehearth for Glass Furnace" and Augsburger U.S. Pat. No. 2,919,297 of Dec. 29, 1959 for "Means of Controlling Electric Currents in a Furnace Forehearth". In Gell U.S. Pat. No. 3,506,769 of Apr. 14, 1970 for "Furnaces for Supplying Molten Glass" there is shown a feeder duct for molten glass in which paired electrodes are arranged in a diagonal relationship to the longitudinal axis of the duct to cause a zig-zag flow of current. Zoned control of Joule effect heating of molten glass in forehearths by sensing current at the downstream electrode of each zone is shown in Stevenson U.S. Pat. No. 4,247,733 of Jan. 27, 1981 for "Electrically Heated Glass Forehearth".
The aforenoted patent disclosures are directed to control of the molten glass temperature longitudinally of the forehearth and thus the flow path to the glass delivery position. In British Pat. No. 1,163,531 by Elemelt Limited, published Sept. 10, 1969 it was recognized that the cross section of the glass in a plane transverse to the length of the forehearth was subject to varying rates of heat exchange in the upper and lower portions and thus tended to have non-uniform temperatures over that cross section. Heat exchange means associated with the upper layer of the glass, gas fired burners and nozzles for introducing cooling air to the free surface of the molten glass were shown with controls for bringing the heating or cooling means into operation as the temperature of the upper portion of the glass stream dictates. The lower layers of the molten glass were heated by Joule effect by passing alternating electric current longitudinally through the glass between electrodes spaced longitudinally along the bottom wall of the forehearth. The heat exchange means cooperating with the upper layers of glass and the electrodes providing the Joule effect heating of the lower layers of glass are segregated into longitudinal zones along the forehearth. A preferred arrangement employs a relatively course adjustment in the upstream zone and a finer adjustment in a downstream zone.
Barkhau et al U.S. Pat. No. 4,389,725 of June 21, 1983 for "Electric Boosting Control for a Glass Forehearth" seeks to further equalize the temperature across a cross section of the conditioning section of a forehearth, that portion immediately preceding the region from which glass is delivered to forming apparatus, by employing longitudinal flow of current along the sidewalls of the conditioning section. It is pointed out that the glass adjacent the sidewalls tends to be cooler than that in the center of the cross section of the forehearth and this tendency can be mitigated by applying controlled current from current sources common to the electrodes on both sidewalls. A temperature sensing means in the glass adjacent a sidewall of the conditioning section is arranged to adjust the current to the electrodes by means of a temperature override circuit which can be set to a desired temperature and a current controller to bring the glass near the sidewall to or nearly to the selected set point.
It has been found that inequalities in the molten glass temperature over the cross section of the conditioning section occur with the arrangement disclosed in the aforenoted U.S. Pat. No. 4,389,725. In order to optimize the state of the molten glass to be issued at the delivery station fed by the forehearth, it is desirable to minimize differences in the temperature of the glass on the opposite sides of the stream flowing to the delivery station.